1. Field of the Invention
The present invention relates to apparatus for electroplating a metal onto a plurality of articles having electrically conductive surfaces, and in particular to an apparatus for exposing each of the articles to the same current densities in an electrolyte plating bath to obtain a uniform plating thickness on the articles.
2. Description of the Prior Art
In the manufacture of certain articles such as electronic circuits or printed wiring boards, a metal is often plated onto electrically conductive surfaces, or selected areas of the surfaces, of the articles. In the case of electronic circuits or printed wiring boards a thin pattern of a conductive material (i.e., titanium and/or palladium) is first formed on one or more surfaces of a substrate, such as a ceramic substrate. The pattern may cover the entire surface, or may define individual circuit paths thereon. The pattern is then plated to a predetermined minimum thickness with a metal, such as copper or gold, to form a conductive layer capable of carrying electrical signals. Plating a metal onto the conductive pattern contemplates immersing the substrates in an electrolyte plating bath containing in solution a salt of the metal to be plated onto the pattern, and applying a voltage between the pattern and an anode within the plating bath to generate a current flow between the pattern and the anode, and through the plating bath, with the pattern as the cathode to plate the metal onto the pattern.
Conventionally, a plurality of substrates are secured within a support rack or plating fixture, a cathodic electrical connection is made with the conductive pattern on the substrates, and the racks with the substrates therewithin are immersed within the plating bath to plate the substrate. With this technique, each substrate within the plating bath is at a different geometric location with respect to the anode therewithin, and the pattern on the substrates are therefore exposed to differing current densities and plating rates. This results in excessive use of the plating metal, since to ensure that a minimum thickness of metal is plated onto each pattern on all of the substrates, thicknesses substantially in excess of the minimum must be plated onto patterns on some of the substrates. When the plating metal is gold, the excessive use thereof adds substantial cost to the plating operation. And while an anode may be geometrically configured to enable all areas of a pattern on a single substrate to be plated to a uniform thickness where one substrate at a time is plated within the plating bath, the savings resulting from minimizing the use of gold would be more than offset by the additional expense of single substrate plating as compared with batch plating.
Another disadvantage of the aforementioned conventional plating technique is that not only the substrates, but also the racks or fixtures for supporting the substrates, must be extended within the plating bath, which increases the risk that contaminants will be introduced into the plating bath requiring disposal or cleansing thereof. Furthermore, frequent manipulation of the fixtures as the substrates are secured therewithin and removed therefrom results in the need for periodic maintenance and repair of the fixtures which, of course, adds expense to the plating operation.